Remove `projectCards` as a fetchable JSON field if Projects v1 is unavailable

### Describe the feature or problem you’d like to solve

`projectCards` is a relic of the Projects (classic) experience and is no longer available in the API [as of April 2025](https://github.blog/changelog/2024-05-23-sunset-notice-projects-classic/#rest-api-timeline). Currently, this field is still shown (as an empty list) in `gh issue/pr` + `--json` calls even when the feature is not available.

### Proposed solution

- Remove `projectCards` field from `gh` commands where were supported if the Projects (classic) feature is not available.
- Let the command error out as it normally would if a user was trying to fetch any other non-existing field.

### Additional context

- This work replaces the need for explicitly checking for field access and returning a special error message, as described in the following issues:
- #11410
- #11411
- #11412
- #11413
- #11414
- #11415
- The majority of users still accessing the field are on older clients (e.g. 2.16), so these changes wouldn't impact them anyway.